MAGNETOCHEMISTRY OF THE TETRAHALOFERRATE(III) IONS - CRYSTAL-STRUCTURE AND MAGNETIC-ORDERING IN [(4-CHLOROPYRIDINIUM)FECL4]2.4-CHLOROPYRIDINIUM CHLORIDE AND RELATED SALTS

The yellow compound of stoichiometry (4-chloropyridinium) 3Fe2Cl9 belongs to the space group P2 1/n with four formula units in the unit cell. Structural analysis shows that the iron is present as the FeX4- ion. Magnetic measurements on single crystals show that the material orders as a canted antiferromagnet at 2.725 K. Replacement of the 4-chloro cation by the 4-bromo analog yields an isomorphous material and shifts the transition temperature to 2.34 K. The phase diagrams of both salts have also been determined. For [(4-chloropyridinium)FeCl4]2·4-chloropyridinium chloride, the bicritical point is at 2.40 K and 7 kOe, and HSF(0)=5.5 kOe. With Hc(0)=56 kOe, α=HA/HE=1. 9×10-2. In the case of [(4-bromopyridinium)FeCl 4]2·4-bromopyridinium chloride, HSF(0)=3 kOe, Hc(0)=42 kOe, α=1.0×10-2, and the bicritical point is at 2.24 K and 4.2 kOe. Crystallographic investigations at 25 K are reported on the isomorphous (4-chloropyridinium)3Fe 2Br9 and on (4-bromopyridinium)3Fe 2Cl1.3Br7.7. Magnetic measurements on single crystals show that (4-bromopyridinium)3Fe2Cl 1.3Br7.7 orders as a canted antiferromagnet at 5.67 K, while (4-chloropyridinium)3Fe2Br9 orders similarly at 7.96 K. These compounds behave approximately as S=5/2, three-dimensional Heisenberg antiferromagnets.